The present invention relates to a hydrocyclone with an inflow region providing tangential inflow for a feed slurry and a separation region following the inflow region with an underflow discharge pipe to carry off heavy materials and an overflow nozzle projecting into the interior of the hydrocyclone.
Generally, a hydrocyclone comprises a cylindrical segment with a tangential inflow (inlet nozzle) and a subsequent conical segment with the underflow nozzle or apex nozzle. The vortex finder or overflow nozzle projects axially from above in the form of an immersion tube into the interior of the cyclone.
The designations “upward(s)” and “downward(s)” in the present description refer to the overflow (specifically lighter and/or finer-grained fraction) and the underflow (specifically heavier and/or coarser fraction). However, the actual positioning of the hydrocyclone largely does not depend on this, so horizontally installed hydrocyclones are also frequently used.
Hydrocyclones are separation units which are used to separate mixtures of solids on the basis of different sinking speeds. In this process, the fractions are not separated fully from one another, and the large differences in the sinking speed are substantiated with greatly varying probabilities of reaching the respective coarse or fine particle outlet.
As a rule, the suspension is fed to the head piece of the cyclone, forced from there into a downward orbit and accelerated into the resulting downward spiral due to the conical taper of the bottom cyclone section. This acceleration and the resulting centrifugal forces create a strong force field that drives all particles that are specifically heavier than the surrounding fluid outwards, while all of the lighter particles are conveyed inwards. The layers close to the core in the overflow stream flowing upwards are detached along the entire downward spiral. The thickened stream discharged at the bottom is called the underflow, and the upwardly discharged stream is designated as the overflow or top flow.
Naturally, the overflow stream contains significantly less solids than the outer streams flowing downwards. In addition, particles with a very low sinking speed have a much higher probability of entering the overflow stream than is the case for the coarse-grained fractions, with the result that the overflow is enriched with fines (relatively in relation to the solids mass). However, the reverse is true in terms of the volume (in mg/l)—in relation to the volume flow discharged, the fines are depleted in the overflow in fine fractions if these fractions have a higher specific weight than the fluid.
Thus, the fines concentration in the underflow increases (often unintentionally) in relation to the fluid volume removed.
In order to prevent this, a wash water cyclone was developed with the aim of creating a barrier water layer (auxiliary sedimentation layer) by means of a lamella and which will make it more difficult for the fines to sediment in the area discharged downwards because of the reduced sedimentation speed. This special hydrocyclone is described in WO 2013/117342.
However, unstable conditions often arise in this hydrocyclone, particularly in the area of the flow reversal in the conical outlet area, which result in strong movement by the vortex in the core area and can thus cause the fractions originally separated to be mixed together again. In addition, individual streams may be discharged wrongly, which can also increase the amount of grains discharged wrongly, if the upward flowing core stream (with fines) and the downward flowing, washed underflow are close to one another.
Additionally, it has been shown that there is an increased amount of wrongly discharged core stream in the underflow, especially if the feed slurry has a high temperature. The problem is aggravated by the addition of wash water, which can also lead to significant dilution in the underflow.